WO1997004624A1

WO1997004624A1 - Circuit arrangement

Info

Publication number: WO1997004624A1
Application number: PCT/IB1996/000659
Authority: WO
Inventors: Hendrik Anton Van Esveld; Petrus Antonius Maria Weerdesteijn
Original assignee: Philips Electronics N.V.; Philips Norden Ab
Priority date: 1995-07-17
Filing date: 1996-07-09
Publication date: 1997-02-06

Abstract

The invention relates to a circuit arrangement for operating a high-pressure discharge lamp, which circuit arrangement is provided with input terminals for connection to a supply source, a first and a second lamp connection terminal for connecting the high-pressure discharge lamp, ignition means for igniting the lamp, which ignition means are provided with a pulse transformer with a primary winding which forms part of a pulse generation circuit and with a secondary winding which is connected with a first end to the first lamp connection terminal and with a second end to an input terminal, and a high-frequency shunt circuit provided with capacitive means connected at one side to the second end of the secondary winding of the pulse transformer and at the other side to the second lamp connection terminal. According to the invention, the high-frequency shunt circuit comprises a low-ohmic impedance of at least 10 Φ.

Description

Circuit arrangement.

The invention relates to a circuit arrangement for operating a high- pressure discharge lamp, which circuit arrangement is provided with input terminals for connection to a supply source, a first and a second lamp connection terminal for connecting the high- pressure discharge lamp, ignition means for igniting the lamp, which ignition means are provided with a pulse transformer with a primary winding which forms part of a pulse generation circuit and with a secondary winding which is connected with a first end to the first lamp connection terminal and with a second end to an input terminal, and a high-frequency shunt circuit provided with capacitive means connected at one side to the second end of the secondary winding of the pulse transformer and at the other side to the second lamp connection terminal.

The circuit arrangement of the kind mentioned in the opening paragraph is known from EP-A-0 507 396 and is suitable for igniting metal halide lamps and high- pressure sodium lamps. The circuit arrangement will often also comprise means for maintaining a stable discharge in the lamp after the lamp has ignited.

Alternatively, the circuit arrangement may form an independent ignition device. This is usually the case when the current through the lamp is stabilized by a separate stabilizer ballast during stable lamp operation, i.e. the stable operating condition of the lamp.

In an ignition process of a high-pressure discharge lamp, the following stages can be distinguished:

- lamp not ignited; the lamp is electrically non-conducting;

- breakdown in the lamp; the lamp becomes electrically conducting, so that the impedance through the lamp shows an abrupt drop; a glow discharge occurs in the lamp; - the run-up of the lamp; the glow discharge gradually changes into an arc discharge; the voltage across the lamp rises, after its abrupt drop owing to the abrupt drop in impedance, to a value accompanying a stable arc discharge in the lamp; the current supply is taken over by the supply source during this.

The types of high-pressure discharge lamps mentioned require a voltage pulse before the lamp ignites. For general applications, the admissible pulse height of the voltage pulse is at most 5,000 V for safety reasons. A consequence of this is that such a lamp cannot re-ignite immediately after extinguishing. Extinguishing of said lamp takes place, for example, owing to a sudden dip in the voltage of the supply source. Re-ignition of the lamp cannot take place until the lamp temperature, in particular that of the gas filling of the discharge vessel, has fallen sufficiently.

Especially when the relevant lamp is provided with a discharge vessel with a ceramic wall, there is the risk of such a dissipation occurring in the discharge vessel, owing to the ignition pulses which occur before the lamp has cooled down sufficiently, that the temperature of the discharge vessel remains substantially constant and the lamp accordingly remains extinguished for a very long time. This may amount to several hours in an incidental case. Not only does the lamp remain extinguished, but is also adversely affected the useful life of the lamp and the ignition means. Another problem in the known circuit arrangement is a bad useful life of the circuit arrangement in the case of ignition of high-pressure discharge lamps of low power rating. This relates to lamps of at most 100 W, in particular below 50 W. Although breakdown does occur in the lamp, there is often no transition from glow to arc discharge which immediately follows it, with the drawback that the circuit arrangement is heavily loaded for a long time.

The invention has for its object to provide a measure for counteracting said drawbacks. In order to be a circuit arrangement according to the invention, a circuit arrangement of the kind mentioned in the opening paragraph is characterized in that the high- frequency shunt circuit comprises a low-ohmic impedance of at least 10 Ω.

Not only does the measure taken prove to be efficacious for counteracting a premature interruption of the cooling-down process of the lamp after extinguishing, but it is surprisingly found that the measure leads to a re-ignition of the extinguished lamp at a higher lamp temperature, and accordingly within a shorter time interval after extinguishing. This surprising result is obtained both in lamps provided with discharge vessels with ceramic walls and in lamps with discharge vessels made of quartz or quartz glass. A ceramic wall is here understood to be a wall formed from a refractory material such as monocrystalline metal oxide (for example, sapphire), densely sintered polycrystalline metal oxide (for example, Al₂O₃, YAG), and polycrystalline densely sintered metal nitride (for example, AIN).

The measure according to the invention also has the result that the ignition time of a low-power lamp is considerably shortened owing to a strongly improved transition from glow discharge to arc discharge. This occurs especially in the case of cold starting of the lamp. The low-ohmic impedance preferably is no greater than 700 Ω, since otherwise there would be a damping and mistuning effect on the high-frequency shunt circuit such that lamp ignition could be adversely affected.

It is noted that the measure according to the invention has no influence on the level of the ignition pulse generated by the pulse generation circuit and applied to the connection terminals of an extinguished lamp. It is true that this pulse height can be influenced by the impedance of the supply conductor between the secondary transformer winding and the lamp connection terminal, but such an influence is only noticeable in the case of an exceptionally great length of the supply conductor. In addition, the supply conductor impedance is only partly of an ohmic nature. The measure according to the invention does not have an effect until after the moment breakdown occurs in the extinguished lamp, in other words when the extinguished lamp changes from an electrically non-conducting to an electrically conducting state. The moment breakdown occurs, the lamp, the high-frequency shunt circuit, and the secondary transformer winding form a closed resonance circuit. The low-ohmic impedance used in accordance with the invention will influence the current and voltage amplitudes occurring in this resonance circuit. The conductor impedances, which moreover have only partly an ohmic character, play a negligible part here.

The low-ohmic impedance is placed in said resonance circuit such that it forms part of the high-frequency shunt circuit only. The low-ohmic impedance could indeed be placed elsewhere in said resonance circuit with a similar result for the ignition behaviour of the circuit arrangement and the connected lamp, but in that case the low-ohmic impedance would also form part of the current circuit through the lamp during stable lamp operation, which would exclusively lead to dissipation.

The input terminals of the circuit arrangement according to the invention may be connected to a usual supply source of 220 V, 50 Hz, for example, via a stabilizer ballast. It is alternatively possible for a square-wave supply voltage to be applied to the input terminals. Another possibility is that the circuit arrangement comprises means for converting a sinusoidal input voltage, for example 220 V, 50 Hz, into a square-wave voltage of, for example, 300 V.

If the circuit arrangement also comprises means for maintaining a stable discharge in the lamp, these means will be arranged between the input terminals and the high-frequency shunt circuit.

The above and other aspects of the invention will be explained in more detail with reference to a drawing in which

Fig. 1 is a diagram of a lamp operating circuit provided with a circuit arrangement according to the invention,

Fig. 2 shows a lamp operating circuit provided with a circuit arrangement according to the invention,

Fig. 3 is a graph showing the lamp temperature after extinguishing and restarting, and

Fig. 4 shows the voltage across the lamp as a function of time during ignition of the lamp from the cold state.

In Fig. 1 , a circuit arrangement according to the invention is provided with input terminals 1 for connecting a supply source, for example an AC voltage source of 220 V, 50 Hz, and a first and a second lamp connection terminal A, B, respectively, to which a lamp L is connected. I are ignition means and II a high-frequency shunt circuit. The circuit arrangement also comprises means III for maintaining a stable discharge in the lamp L after ignition. Of the means I, a primary winding 21 of a pulse transformer 2, a breakdown element 3 acting as switching means, and a capacitor 4 acting as capacitive means form a pulse generation circuit. The capacitive means 4 provide a periodic charge change in the form of charge reversal via transformer 2 and breakdown element 3. The transformer 2 comprises a primary winding 21 as a primary part and a secondary winding 22 as a secondary part. The secondary winding 22 is connected with a first end 22a to the first lamp connection terminal A, and with a second end 22b to an input terminal 1. The pulse generation circuit is connected with a resistor 6 to the second lamp connection terminal B, which serves to safeguard that the circuit arrangement will become operative after connection of the supply source. The high-frequency shunt circuit II is provided with a capacitor C_p as capacitive means which guarantee a defined current path in the case of breakdown of the lamp L.

Capacitor C_p is connected at one side to the second end 22b of the secondary winding 22 of the pulse transformer 2 and at the other side to the second lamp connection terminal B. The high-frequency shunt circuit II also comprises a low-ohmic impedance 10. If the circuit arrangement comprises no means for maintaining a stable discharge, the input terminals are formed by the points 1'.

The embodiment described is suitable inter alia for use as a lamp operating circuit in the form of a high-frequency switch mode power supply (smps) supplying a square- wave voltage to the lamp. The configuration described realizes substantially a voltage doubling across the breakdown element 3 without the need of further aids. This renders it possible to choose the breakdown voltage of the breakdown element 3 such that with certainty no breakdown can occur during stable lamp operation. A practical realization of the lamp operating circuit described was tested for the ignition of a metal halide lamp of a power rating of 70 W. The lamp operating circuit was realized in the form of a stabilizer ballast, i.e. a self-inductance suitably connected to the supply source AC voltage of 220 V, 50 Hz, followed by the circuit arrangement for igniting the lamp. The circuit arrangement for igniting the lamp had a construction corresponding to that of a type HZN150S igniter, make Bag Thurgi, to which a low-ohmic impedance was added in the high-frequency shunt circuit. The low-ohmic impedance 10 had a value of 300 Ω and was formed by a wire resistor. A carbon resistor is also very suitable for use as the low- ohmic impedance 10. A metal film resistor is less suitable for use as the low-ohmic impedance 10 for reasons of durability. The capacitive means, i.e. capacitor C_p, had a value of 2 μF.

Immediately after connection of the lamp operating circuit to the supply source, a sinusoidal voltage of 220 V will be across the circuit arrangement. This has the result that the breakdown element 3 breaks down the moment the voltage across capacitor 4 has reached the breakdown voltage of breakdown element 3, and a charge reversal across capacitor 4 takes place via primary winding 21 and self-induction coil R_s. This yields a pulse height of 4 kV at secondary winding 22.

Fig. 2 shows a modification of the ignition means I, where components corresponding to those of Fig. 1 have the same reference numerals. The low-ohmic impedance of the high-frequency shunt circuit here also forms part of the charging circuit of capacitor 4 of the pulse generation circuit. As a result, the value of resistor 6 may be chosen to be somewhat lower compared with the configuration of Fig. 1.

Fig. 3 is a graph showing the relation between temperature T, plotted on the vertical axis, of a discharge vessel of a high-pressure discharge lamp and time t plotted on the horizontal axis. This relates to test results of a metal halide lamp of 70 W power rating as described here.

Line portion A gives the temperature T during stable lamp operation. A voltage dip takes place at moment to, so that the lamp extinguishes. The temperature T of the discharge vessel subsequently drops in accordance with line portion B. In a first case, the lamp is so connected to the circuit arrangement that no re-ignition takes place. Temperature T in that case continues falling continually in accordance with line portion C.

If the lamp has a quartz glass discharge vessel and is connected to a circuit arrangement according to the prior art, re-ignition will take place at moment t,. The temperature T of the discharge vessel will then rise in accordance with line portion D up to the level belonging to a stable discharge. Re-ignition takes place within a period of 600 s = 10 minutes after extinguishing. If the lamp has a discharge vessel with a ceramic wall and the circuit arrangement is a circuit arrangement according to the prior art, conduction will occur in the lamp at moment t₃. The conduction thus arising, however, does not lead to the establishment of a stable discharge, but only to a dissipation such that the temperature T remains substantially constant as shown with line portion E.

Finally, the case is considered where the lamp is connected to the circuit arrangement of Fig. 1. Ignition is then found to occur at moment t₃, whereupon the temperature T of the discharge vessel rises in accordance with line portion F up to the level accompanying a stable discharge. This is found to be the case both for the lamp having the quartz glass discharge vessel and the lamp having the discharge vessel with a ceramic wall.

In the tests described with the lamp connected to the circuit arrangement of Fig. 1 , the low-ohmic impedance 10 was varied between 10 Ω and 700 Ω. The gradient of temperature T shown in Fig. 3 with line portion F is realized for values of the low-ohmic impedance 10 of between 25 Ω and 400 Ω.

Lower and higher values of the low-ohmic impedance will lead to slight shifts in time of line portion F. When the impedance value becomes lower than 10 Ω, the influence of the impedance on the re-ignition behaviour becomes so small that the temperature T follows the line portion E or D, depending on the discharge vessel material. With an impedance value chosen above 700 Ω, there is a risk of high-frequency interferences occurring in the means for maintaining a stable discharge, or even propagating themselves to the supply source through the input terminals. In addition, the peak current is limited in the closed tuned circuit of lamp and high-frequency shunt circuit to such an extent that there is a risk of the glow discharge not changing into an arc discharge.

In a further practical realization of the circuit arrangement according to the invention, the ignition of a low-power lamp was investigated. The lamp was a 39 W metal halide lamp provided with a discharge vessel with a ceramic wall. The lamp was ignited in the cold state, during which a low-ohmic impedance of 300 Ω was present in the high-frequency shunt circuit. A stable discharge arises in the lamp within 30 ms, as shown in Fig. 4a, where the voltage across the lamp V_u is plotted against time.

The circuit arrangement also comprises electronic means for maintaining the stable discharge, the voltage being commutated with 100 Hz, as is evident from the periodic commutation of the voltage across the lamp V_la represented in Fig. 4a. For comparison, the same lamp was ignited in the cold state by means of the same circuit arrangement, but with the low-ohmic impedance being left out. The ignition behaviour of the lamp is shown in Fig. 4b. It is evident from the Figure that a large number of ignition pulses over a period of 90 ms do not lead to the start of an arc discharge in the lamp.

Claims

CLAIMS:

1. A circuit arrangement for operating a high-pressure discharge lamp, which circuit arrangement is provided with input terminals for connection to a supply source, a first and a second lamp connection terminal for connecting the high- pressure discharge lamp, ignition means for igniting the lamp, which ignition means are provided with a pulse transformer with a primary winding which forms part of a pulse generation circuit and with a secondary winding which is connected with a first end to the first lamp connection terminal and with a second end to an input terminal, and a high-frequency shunt circuit provided with capacitive means connected at one side to the second end of the secondary winding of the pulse transformer and at the other side to the second lamp connection terminal, characterized in that the high-frequency shunt circuit in addition comprises a low-ohmic impedance of at least 10 Ω.

2. A circuit arrangement as claimed in Claim 1 , characterized in that the low-ohmic impedance has a value of at most 700 Ω.